Fluid filtration, e.g., gas filtration or liquid filtration, is important in a wide variety of industrial processes, including in the manufacture of electronic devices. Air filtration, for example, is very important in manufacturing semiconductors. Semiconductors are manufactured in ultra-clean manufacturing facilities called “clean rooms”. Unlike ordinary rooms, clean rooms are substantially free of particulate contaminants. Typical clean rooms can have less than 100 particles per cubic foot. If the air in a clean room is not substantially free of particulate contaminants, the particulate contaminants in the air can decrease semiconductor yields. For example, contaminants in the air of a clean room can deposit on an unprotected circuit of a semiconductor potentially short-circuiting and damaging the semiconductor. Damaged semiconductors are usually reworked or discarded as scrap.
Conventional air filters for clean rooms use filter media made from glass fibers. While glass fiber filter media can be capable of filtering particulate contaminants from an incoming air stream, glass fiber filter media itself can produce contaminants which can lower semiconductor yields. For example, conventional glass fiber filter media contains borosilicate glass, which can produce boron-containing contaminants. Fluorine gas (e.g., released from etching processes) and water in the air of a clean room can combine to react with the borosilicate glass fibers in the filter media and produce airborne, boron-containing contaminants such as silicon boride. These airborne, boron-containing contaminants can decrease semiconductor yields by settling on the semiconductors during processing. Silicon boride contaminants, for example, can degrade the electrical properties of semiconductors and consequently decrease semiconductor yields. Since the air in a clean room must be filtered, most semiconductor process engineers simply accept airborne boron contamination as inevitable and employ additional process steps to compensate for the contamination. For instance, a buffered HF (hydrogen fluoride) clean-and-etch step can be added to remove some of the boron contamination from the surface of a semiconductor.
Filtering the clean room air with a filter medium substantially free of boron may reduce the amount of boron contaminants in a clean room. However, conventional filters incorporating substantially boron-free filter media, such as polymeric filter media, may not be advantageous. A filter medium is typically pleated in order to increase the filter surface area for a predefined envelope (e.g., a space defined by a housing or frame). When a polymeric filter medium (e.g., a filter medium formed from a polymeric material such as polymeric fibers) is pleated, the medium can have a tendency to “spring back” in the pleat tip region, resulting in a pleat tip region with a conventional bulbous shape. A pleated filter medium having conventional bulbous-shaped pleat tip regions can disadvantageously have a low number of pleats per unit of length, exhibit high differential pressure when filtering a fluid and/or have a diminished structural stability. Further, due to the “spring back” properties of many polymeric filter media, such media can be difficult to pleat.
FIG. 1 shows a portion of a filter element 20, which includes a pleated filter medium 11 having a plurality of upstream conventional bulbous-shaped pleat tip regions 11(a) and downstream conventional bulbous-shaped pleat tip regions 11(b). A contaminated fluid stream 21 flowing towards the upstream spaces 12(a) between the pleats of the pleated filter medium 11 may be impeded by the upstream conventional bulbous-shaped pleat tip regions 11(a). The spaces 12(a) between the adjacent upstream conventional bulbous-shaped pleat tip regions 11(a) are relatively narrow and may bottleneck the flow of the contaminated fluid stream 21 into the pleated filter medium 11. Eventually, the contaminated fluid stream 21 passes through the pleated filter medium 11 forming a purified fluid stream 22. The purified fluid stream 22 can then flow in the downstream spaces 12(b) between the downstream pleat tip regions 11(b) of the pleated filter medium 11, before exiting the pleated filter medium 11. As the purified fluid stream 22 flows downstream, the downstream conventional bulbous-shaped pleat tip regions 11(b) can impede the flow of the purified fluid stream 22 out of the pleated filter medium 11. The spaces 12(b) between the adjacent downstream conventional bulbous-shaped pleat tip regions 11(b) are relatively narrow, and may bottleneck the flow of the purified fluid stream 22 out the pleated filter medium 11. The fluid stream lines 17 through the filter element show the bottlenecks at the upstream and downstream pleat tip regions. These bottlenecks greatly increase the pressure drop across the filter element. Consequently, greater upstream pressure may be required to force the contaminated fluid stream 21 past the upstream conventional bulbous-shaped pleat tip regions 11(a) and the purified fluid stream 22 past the downstream conventional bulbous-shaped pleat tip regions 11(b). Unfortunately, greater upstream fluid pressures require greater energy to compress the contaminated fluid stream.